Waterproof they are not. Water resistant would also be a stretch. Some bikes have some serious issues with water (giant ebikes and corroding battery contacts), others, seem to do ok. The simple truth is that other then electrical connectors, the motors are the most vulnerable part. Water will find it's way in, and it will toast the bearings sooner or later. That being said, I'm usually surprised by a handful of people that ride these in wet conditions and how few issues they have (uk riders)... but for the most part, they work about as well as an early cell phone used regularly in a rain storm.
The people that have the most issues, are the people that hose wash their ebikes.